Investment material for precision casting



United States Patent INVESTMENT MATERIAL FOR PRECISION CASTING Claude H.Watts, Lindhurst, Ohio, assignor to Pre-Vest, Inc.

No Drawing. Application October 5, 1959 Serial No. 844,432

4 Claims. (Cl. 106-383) This invention relates to an improved bondedrefractory investment material. Its use is primarily intended for thelost wax or lost pattern method of precision investment casting.

This application is a continuation-in-part of application Serial No.627,825, filed December 12, 1956, entitled, Investment Material forPrecision Casting.

In the parent application identified above, there is a full and completeteaching of a new and novel investment material composed of gradedrefractory bound by the reaction products of fused magnesium oxide andan acidified sodium silicate solution. Reference is made to thespecification of that application for a full disclosure of the means ofpracticing the invention and the superior results to be expected byfollowing that teaching.

In the teaching of the parent application, it was disclosed that theammonia radical could be used to modify the setting reaction. At thetime the specification for the parent application was written, it wasbelieved that the preferable means for modifying the primary binder wasto neutralize a portion of the phosphoric acid of the formula with aquaammonia. It was stated that mono ammonium phosphate was theoreticallyuseful, but not to be preferred.

However, it has since been discovered that the use of mono ammoni tingreaction induced by the unique nature 0 'comml'ciaf fused magnesiumoxide. In the laboratory, where a carefully graded magnesium oxide isalways available, and often with portions of a commercial shipment ofthe fused magnesium oxide, the grain size distribution is uniform andthe use of liquid aqua ammonia is quite satisfactory. Laboratoryconditions and ideal commercial products are more the exception than therule. It has been found that, regardless of the effort made to obtainexactly uniform grain size distribution, commercial shipments of fusedmagnesium oxide will vary widely in the amount of fines in the product.The specification of the parent application to which this specificationis appended, teaches the fact that graded fused magnesium oxide isemployed for the purpose of aiding in the control of the setting time ofthe binder. If the proper grain size distribution is not available, thenthe setting time will vary unless compensated for by appropriate buffermeans. It has been found that the mono ammonium phosphate is the idealbuffer in such situations and when used in this form the ammoniumradical will be available to control the setting reaction.

The ammonium radical is employed to temper or slow the setting reaction.If the magnesium oxide grain distribution is exactly as specified, it isgenerally not necessary to employ any bufiering action by the ammoniumradical. Whenever the fines distribution becomes excessive, an amount ofthe ammonium radical is employed to temper the reaction and bring thesetting time back to its desired speed. Hence, there is theoretically noREEREHCE I U C011- 0 variables of the set "ice lower limit to the amountof the ammonium radical which may be employed beneficially, but actualpractice has indicated that the lower practical range for aqua ammoniais the three percent amount specified in the parent application, or itsequivalent in the form of mono ammonium phosphate.

The value of the use of mono ammonium phosphate is that after the powdermix of the graded refractory material and the graded fused magnesiumoxide has been blended properly for shipment, its characteristic withrespect to the liquid phase of the binder may be checked in thelaboratory. If the setting time is too fast, it then becomes a simplematter of control for the manufacturer to incorporate an appropriateamount of powdered mono ammonium phosphate into the dry materials to theextent that a properly mixed liquid phase will produce just exactly theright setting. Alternatively, the shipment may contain instructions toadd particular amount of the mono ammonium phosphate to the liquid phaseif the ultimate user is better equipped to handle material in thismanner. The greatest advantage of this species development of theinvention is in the discovery that the powder mix may be tempered at thefactory before shipment in order to assure the exact settling time ofthe investment slurry when wetted with the standard acidified silicatesolution. Thus, the control laboratories of the investment producer maybe certain of the reaction of the material in production withoutsupervising the control of the material in the consumers operation.

Specifically, it is the object of this invention to modify an aci oxidebinder by the use of the ammonium radical carrim'e form of mono ammoniumphosphate.

Another object of this invention is to produce an extended settingperiod of such a binder while retaining the desired investmentcharacteristics.

A still further object of this invention is to cause the modificationcharacteristic to take place as the acidified silicate is mixed with therefractory to be bound.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims.

In this specification it should be noted that the term refractory is notmeant to include materials rapidly reactable with acid ions. Magnesiumoxide is used as a refractory for high-temperature ovens, but its use inthe improved investment of this invention is to react with the acidifiedsilicate to produce a complex binder; and it is neither retained asmagnesium oxide, nor considered as the refractory to be bound.

The refractory material will preferably be a blend of silica ofcommercial grade wherein about half is 200 mesh and the balance isdivided into two parts of 30 mesh and one part 50 mesh, thus thedivision is approximately 50 percent of 200 mesh, 30 percent of 30 mesh,and 15 percent of 50 mesh. The commercial grades of such refractoryprovide a tolerance in the grade percentages and therefore thesepercentages are only close approximations.

The basic concept of the parent application is in the or pref- ExAMmq' il l i i A specific example of the preferred embodiment of this alternatemethod as used in actual production operations is as follows:

An acid solution is prepared comprising:

725 lbs. of 75 percenwhphosphoric acid) 826 lbs. of water A sodiumsilicate solution is prepared comprising:

150 lbs. of 89331112133568: 290 lbs. of water The liquid bi is reparedusing:

3 lbs. 4 oz. of the acid solution 5 lbs. 13 oz. of the sodium silicatesolution and 11 lbs. 2 oz. additional water An investment slurry is madeup by adding to the above liquid binder, 100 lbs. of powder materialcomprising 93 percent graded refractory powder into which has been mixed4 percent fused magnesium oxide and 3 percent mono ammonium phosphate.

In the following examples either 75 percent ortho phosphoric acid or 85percent ortho phosphoric acid may be used so long as the actual amountof 100 percent ortho phosphoric acid remains the same. For example 5.15cc. of 75 percent phosphoric acid, specific gravity 1.58 is equivalentto 4.2 cc. of 85 percent phosphoric acid, specific gravity 1.71.

For example a formula calling for 725 lbs. of 75 percent orthophosphoric acid would only require 640 lbs. of 85 percent orthophosphoric acid to obtain for practical purposes the equivalent amountof ortho phosphoric acid.

The following examples are given to illustrate the range ofconcentrations of liquid binder constituents that are useful in thepractice of this invention, and also the variations that may be employedin preparation of the binders:

Powder and liquid mixed in the proportions of about 23 parts liquid byvolume to 100 parts powder by weight.

Example 11 Liquid:

5 cc. of sodium silicatespecific gravity 1.32 4 cc. of 85 percent H POspecific gravity 1.58 91 cc. of water Powder:

2 parts by weight fused magnesium oxide 1 part by weight mono ammoniumphosphate 97 parts by weight graded refractory Powder and liquid mixedin the proportions of about 23 parts liquid by volume to 100 partspowder by weight.

Example III Liquid:

cc. of sodium silicatespecific gravity 1.32 10 cc. of 85 percent H POspecific gravity 1.58

70 cc. of water Powder:

'8 parts by weight fused magnesium oxide 3 parts by weight mono ammoniumphosphate 89 parts by weight graded refractory Powder and liquid mixedin the proportions of about 23 parts liquid by volume to 100 partspowder by weight,

Calculation from the above examples will show that 23 parts of theliquid by volume of Examples I and II will contain slightly over 20parts of water by weight added as water, exclusive of any water portionin the sodium silicate and acid. Also, from Example III, the water isabout 16.0 parts by weight. Therefore, it may be said that the wateradded to the dry ingredients is in the range of about 15 to 20 parts byweight.

These examples are given to make clear the intent and extent of theuseful material range. The experienced artisan will recognize that theconcentrated silicate cannot be acidified and thereafter diluted, butrather the acidification must take place in the dilute condition.

Further, the mono ammonium phosphate will be properly used whether addedto the liquid phase or the powder phase. As a practical matter it willusually be blended into the powder phase by the supply house, but suchneed not be the case. The result is the same in either case.

Further, the examples are set out with commonly available materials. Itis the active component that takes part in the reaction, and hence onemay use reagents of other specific gravity, for example, and merelyadjust the ranges to provide the same absolute amounts.

Although the invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

What is claimed is:

1. A refractory investment consisting essentially of:

Parts by weight Sodium silicatespecific gravity 1.32 (6.75 sodium oxideto 25.3% silicon dioxide) 1.455.8 Ortho phosphoric acid, %-specificgravity 1.71 1.5-3.75 Graded refractory 89-97 Mono ammonium phosphate1-3 Fused magnesium oxide 2-8 Water to make a workable slurry in a rangeof about 15 to 20 parts by weight,

said graded refractory being substantially inert to rapid reaction withthe balance of the ingredients and their reaction products.

1.71 1.53.75 Graded refractory 89-97 Mono ammonium phosphate 1-3 Fusedmagnesium oxide 2-8 Water to make a workable slurry in a range of about15-20 parts by weight,

thereafter diluting together the sodium silicate and acid to provide anacidified sodium silicate solution, separately mixing the fusedmagnesium oxide throughout the refractory, and finally bringing togetherthe acidified sodium silicate solution, the refractory-magnesium oxidemixture, and the mono ammonium phosphate by mixing to a pourable slurrycondition, whereby the magnesium oxide and the acid ions react to form amagnesium phosphate bond and the silicate is caused to jell uniformly asan auxiliary bond in a complex green and fired strength relationship,said graded refractory being substantially inert to rapid reaction withthe balance of the ingredients and their reaction products.

References Cited in the file of this patent UNITED STATES PATENTS FeaginSept. 12, 1950 Watts Apr. 13, 1954 Moore et a] June 15, 1954 HenricksJune 29, 1954 Henricks Apr. 7, 1959

1. A REFRACTORY INVESTMENT CONSISTING ESSENTIALLY OF: PARTS BY WEIGHTSODIUM SILICATE-SPECIFIC GRAVITY 1.32 (6.75 SODIUM OXIDE TO 25.3%SILICON DIOXIDE) 1.45-5.8 ORTHO PHOSPHORIC ACID, 85%-SPECIFIC GRAVITY1.71 15.-3.75 GRADED REFRACTORY 89-97 MONO AMMONIUM PHOSPHATE 1-3 FUSEDMAGNESIUM OXIDE 2-8 WATER TO MAKE A WORKABLE SLURRY IN A RANGE OF ABOUT15 TO 20 PARTS BY WEIGHT, SAID GRADED REFRACTORY BEING SUBSTANTIALLYINERT TO RAPID REACTION WITH THE BALANCE OF THE INGREDIENTS AND THEIRREACTION PRODUCTS.